(1) Field of the invention
The present invention relates to a vector control apparatus for a speed adjustable induction motor using a slip frequency control method. The vector control apparatus is so arranged as to compensate for variation in an exciting inductance.
(2) Description of the Background Art
Various types of a vector control apparatus for an induction motor have been proposed in which both a secondary magnetic flux and secondary current are controlled with no mutual interference.
Such a vector control as described above is a method in which in a case of a three-phase induction motor current and magnetic flux are handled as vectors in two-perpendicular d-q coordinate system (cartesian coordinate system) which is rotated at the same speed as a rotating magnetic field by means of a power supply angular frequency and the result of calculation of the vectors is converted into a current instruction value of each phase of the three-phase power supply. Such a vector control as described above is exemplified by a U.S. Pat. No. 5,136,228 issued on Aug. 4, 1992.
However, since, in a previously proposed induction motor vector control apparatus which achieves the above-described control method, no consideration is given to a field control, a calculation of correction for a slip is executed with an exciting current i.sub.1d constant, i.e., i.sub.1d =.lambda..sub.2d /M. In a region of field control, therefore, an accurate calculation of a set value of a slip angular frequency cannot be made. An exciting inductance M' of the induction motor is varied according to a frequency and exciting current. Thus, if the exciting inductance is controlled with a ration of M' to I.sub.o (exciting current) constant, an accurate control of torque becomes impossible. Especially, since M' variation in a constant output region is large, an accuracy of torque at the constant output may be reduced.
Although an excitation instruction .lambda..sub.2d */M* can be controlled to be constant in a constant torque range but is controlled so as to be inversely proportional to a motor angular frequency .omega..sub.r in a constant output range. Consequently, the exciting inductance M is largely varied in the constant output range in which the field control is carried out due to a magnetic saturation characteristic of an iron core of the induction motor and an accuracy of torque is deteriorated.